Solid state spark ignition circuit with automatic shut-off

ABSTRACT

An automatic fuel ignition circuit including a capacitive voltage doubler network for controlling the conductivity of a normally non-conducting silicon controlled rectifier which is serially connected with a primary winding of an ignition transformer to provide a shunt discharge path for one of the capacitors of the voltage doubler network such that the capacitor is periodically discharged over the winding of the ignition transformer inducing voltage pulses in a secondary winding of the ignition transformer; the secondary winding of the ignition transformer being connected in series with a pair of ignition electrodes which are positioned adjacent a fuel outlet such that ignition sparks for igniting the fuel are produced between the ignition electrodes whenever voltage pulses are induced in the secondary winding.

United States Patent 1 1191 Dietz et al.

[ SOLID STATE SPARK IGNITION CIRCUIT WITI-I AUTOMATIC SHUT-OFF [75] Inventors: Gerald Edward Dietz; Russell Byron Matthews, both of Goshen, Ind.

[73] Assignee: Johnson Service Company,

Milwaukee, Wis.

[22] Filed: Nov. 16, 1972 [21] Appl. No.: 307,077

1521 u.s.c1 431/74,3l7/96, 431/264 51 1m ..c1. F23q 3/00 58 Field ofSearch 431/74, 66, 67, 264;

[111 3,806,305 Apr. 23, 1974 Primary Examiner-Edward G. Favors v Attorney, Agent, or FirmJohnson, Dienner, Emrich, Verbeck & Wagner 5 7] ABSTRACT An automatic fuel ignition circuit including a capacitive voltage doubler network for controlling the conductivity of a normally non-conducting silicon controlled rectifier which is serially connected with a primary winding of an ignition transformer to provide a shunt discharge path for one of the capacitors of the voltage doubler network such that the capacitor is periodically discharged over the winding of the ignition transformer inducing voltage pulses in asecondary winding of the ignition transformer; the secondary winding of the ignition transformer being connected in series with a pair of ignition electrodes which are positioned adjacent a fuel outlet such that ignition sparks for igniting the fuel are produced-between the ignition electrodes whenever voltage pulses are induced in the 7 Claims, 1 Drawing Figure [56] l I References Cited UNITED STATES PATENTS 3,384,440 5/1968 Mayer 431/66 3,311,759 3/1907 Remy ....317/96X 3,457,456 7/1969 Dietz 3l7/96X sewndmy i? "cf" IF-D/ 1 R/ Z4VAC IIOVAC SOLID STATE SPARK IGNITION CIRCUIT WITH AUTOMATIC SHUT-OFF BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to electronic fuel ignition systems, and more particularly to an automatic fuel ignition circuit for automatically providing ignition and re-ignition of a gaseous fuel.

Description of the Prior Art One known fuel ignition circuit which provides automatic ignition and re-ignition of a gaseous fuel is disclosed in the US. Pat. No. 3,377,125 of Robert J. Zielinski. The fuel ignition circuit employs an ignition transformer having a primary winding and a secondary winding and relaxation oscillator means for producing a pulse of current through the primary winding of the ignition transformer for each cycle in which operation of the relaxation oscillator is initiated.

The secondary winding of the ignition transformer is connected in circuit with a pair of ignition electrodes. Each time a current pulse is provided in the primary winding, a corresponding voltage pulse is produced in the secondary winding which voltage pulse is applied to the ignition electrodes to generate ignition sparks in the region in which a gas flame is to be produced.

The relaxation oscillator includes a normally nonconducting silicon controlled rectifier having an anodecathode circuit connected in a series circuit with the primary winding of the ignition transformer across a power circuit. The power circuit includes an input transformer connected to a 120 volt AC line voltage source and rectifier means which derives a DC voltage from the AC line voltage.

A first capacitor, connected in parallel with the series circuit, is charged by the rectified voltage whenever the SCR is non-conducting. A second capacitor having a charging time constant that is greater than that of the first capacitor is also charged by the rectified voltage for effecting turn-on of the silicon controlled rectifier when the second capacitor has charged to a preselected voltage. I

The turn-on of the silicon controlled rectifier enables the first capacitor to discharge through the primary winding of the ignition transformer, effecting the generation of art-ignition spark at the ignition electrodes for igniting the gaseous fuel.

In many prior art automatic fuel ignition circuits, such as the one disclosed in the referenced patent to Zielinski, a 120 volt AC source is required to provide sufficient capacitor discharge current over the primary winding of the ignition transformerfor effecting generation of sparks between the ignition electrodes connected to the secondary winding of the transformer.

SUMMARY OF THE INVENTION The present invention provides an improved automatic fuel ignition circuit which is capable of producing ignition sparks when energized from a 24 volt AC source.

In accordance with one embodiment of the invention, the automatic fuel ignition circuit includes an electronic igniter circuit and an associated control circuit operable to produce sparks between a pair of ignition electrodes for igniting a gaseous fuel emanating from a fuel outlet.

The control circuit includes a voltage doubler network having first and second capacitors and separate unidirectional charging paths for the capacitors connected across a source of an AC voltage to enable the first capacitor to charge to the magnitude of the applied AC voltage during a first half cycle of the AC voltage and the second capacitor to charge to twice the magnitude of the AC voltage during the second half cycle of the AC voltage.

The control circuit further includes the primary winding of an ignition transformer and a normally nonconducting silicon controlled rectifier which are connected in parallel with the second capacitor.

When the second capacitor becomes charged, the silicon controlled rectifier is rendered conductive during the next half cycle of the AC voltage supplied to the fuel ignition circuit, providing a shunt discharge path for the second capacitor over the primary winding of the ignition transformer. Accordingly, capacitor discharge current flowing over the primary winding whenever the silicon controlled rectifier is in a conductive state induces a voltage pulse in a secondary winding of the ignition transformer which is connected in circuit with the ignition electrodes in the igniter circuit.

The voltage pulses induced in the secondary winding are applied to the ignition electrodes producing ignition sparks between the ignition electrodes for igniting the gas. I

The fuel ignition circuit further includes a flame detector circuit which is enabled whenever the gas is ignited to prevent the silicon controlled rectifier from conducting, such that spark generation is terminated.

Thus, the automatic fuel ignition circuit of the present invention, which employs a voltage doubler network, effectively doubles the amount of capacitor discharge current provided for a given energizing voltage and correspondingly, the amplitude of the voltage pulses induced in the secondary winding of the ignition transformer for the purpose of spark generation. Accordingly, a lower energizing voltage can be used for the fuel ignition circuit to produce ignition sparks.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the single FIGURE which comprises the only drawing of the disclosure, the automatic fuel igni- 'tion circuit 10 provided by the present invention includes an electronic igniter circuit 11 and an associated control circuit 12, operable when energized to produce sparks between a pair of ignition electrodes 13 and 14 for igniting gaseous fuel emanating from an outlet 15 of a conduit 16 of a gas burner apparatus 17. The gas burner apparatus 17 may include a regulating valve 18 for controlling the flow of gas to the conduit 16. Whenever the valve 18 is open to permit the fuel to emanate from the outlet 15, the igniter circuit 11 will produce ignition sparks as long as the fuel is unlit.

The automatic fuel ignition circuit 10 is energized by electrical power supplied over input terminals 21 and 22 of the fuel ignition circuit 10, which are connectible to an AC voltage source. One of the input terminals 21 is connected to ground or a point of reference potential for the ignition circuit 10.

In accordance with a feature of the invention, the magnitude of the energizing voltage may be lower than that required in many prior art electronic fuel ignition circuits.

Accordingly, a low power AC source, providing an AC signal of 24 volts, for example, may be used to energize the fuel ignition circuit 10.

Alternatively, the fuel ignition circuit 10 may be powered from a standard 10 volt 60 Hertz AC voltage source through the use of a voltage divider network 23. The voltage divider network includes a pair of series connected resistors R1 and R2 which may be connected across the 120 volt source, and a pair of output terminals 210 and 22a of the network 23 which may be connected to the input terminals 21 and 22 respectively, of the fuel ignition circuit 10 to supply an AC voltage of an amplitude of approximately 24 volts to terminals 21 and 22.

The control circuit 12 includes a voltage doubler network, indicated generally at 24, comprised of a pair of capacitors C1 and C2, the primary winding 26 of an ignition transformer T1 and a controlled switching device 32, which may be a normally non-conducting silicon controlled rectifier, such as the type Cl06-B silicon controlled rectifier which is commercially available from General Electric Co.

The control circuit 12 provides a first unidirectional charging path for capacitor C l of the voltage doubler network 24 from the ungrounded input terminal 22 over a diode D1 and capacitor C1 to the grounded input terminal 21. A second unidirectional charging path for capacitor C2 of the voltage doubler network 24 is provided from the grounded terminal 21 through capacitor C1, capacitor C2, a diode D3 and a resistor R3 to the ungrounded terminal 22.

The primary winding 26 of the ignition transformer T1 and the anode-cathode circuit of the silicon controlled rectifier 32 are connected in series across capacitor C2 providing a normally open shunt discharge path. The gate of the silicon controlled rectifier 32 is connected over a resistor R4 to the cathode of the silicon controlled rectifier 32 at point 33. The gate of the silicon controlled rectifier 32 is also connected over resistor R5 and reverse connected diode D2 to the junction of diode D3 and resistor R3 at point 38.

As will be described in more detail hereinafter, through voltage doubler action, capacitor C2 is permitted to charge to twice the energizing voltage applied to input terminals 21 and 22 of the circuit. Capacitor C2, when charged, provides a predetermined negative potential at the cathode of the silicon controlled rectifier 32 causing silicon controlled rectifier to become conductive when the energizing signal applied to the input terminals 21, 22 renders the gate of the silicon controlled rectifier 32 positive relative to the cathode. Consequently, when the silicon controlled rectifier 32 becomes conductive, a shunt discharge path is provided for capacitor C2 through the primary winding 26 of the ignition transformer T1, and the capacitor discharge current which flows over the primary winding 26 induces a voltage pulse in the secondary winding 27 of the ignition transformer T1. 1

The secondary winding 27 of the ignition transformer T1, which comprises the igniter circuit 1 1 is connected in a series unidirectional DC circuit path from the ungrounded terminal 22 of the fuel ignition circuit over resistor R3, resistors R6 and R7, a diode D5 and the transformer winding 27 to one of the ignition electrodes 13. A capacitor C3 is connected in parallel with the series connected resistors R6 and R7. A further capacitor C4 is connected between the junction of the cathode of diode D5 and a terminal 31 of the transformer winding 27 and ground.

Thus, voltage pulses induced in the'secondary winding 27 of the ignition transformer T1 when capacitor C2 is discharged through the primary winding 26 of the transformer T1, are applied to the ignition electrodes 13 and 14, generating sparks for igniting the gaseous fuel emanating from the outlet 15.

When the fuel is ignited, a flame bridges the electrodes l3 and 14, lowering the resistance between the electrodes 13 and 14 permitting current to flow over the unidirectional DC path extending from the ungrounded circuit terminal 22, resistors R3, R6 and R7, diode D5, winding 27 and electrodes 13 and 14 to ground.

The fuel ignition circuit 10 further includes a flame detector circuit 36 including a further controlled switching device 37, embodied as a thyristor which may be the type 2N6028, which is commercially available from General Electric Co. The thyristor 37, which is normally non-conducting, has an anode connected to the cathode of diode D3 at point 38 and a cathode connected at point 33 to the junction of the anode of diode D3 and the cathode of the silicon controlled rectifier 32 to provide a normally open shunt path across diode D3. A control electrode of the thyristor 37 is connected to the junction of resistors R6 and R7.

Whenever the fuel is ignited and a flame bridges the gap between the ignition electrodes 13 and 14, current flow over resistors R6 and R7 triggers the thyristor into conduction. When conducting, the thyristor 37 provides a short circuit across diode D3, effectively short circuiting the cathode to gate circuit of the silicon con trolled rectifier 32, preventing the silicon controlled rectifier 32 from conducting and preventing further discharge of capacitor C2. Consequently, spark generation will be terminated.

OPERATION OF FUEL IGNITION CIRCUIT For purposes of illustration of the operation of the automatic fuel ignition circuit 10, it is assumed that the silicon controlled rectifier 32 and the thyristor 37 are both non-conducting and that capacitors C1 and C2 of the voltage doubler circuit 24 are discharged.

When the AC energizing voltage, such as a 24 VAC 60 HZ voltage, is supplied to the ignition circuit 10 over input terminals 21, 22, and the ungrounded terminal 22 becomes positive relative to the grounded input terminal 21, capacitor C1 of the voltage doubler network 24 will become charged over a circuit path extending from terminal 22 over diode D1 and capacitor C1 to the grounded terminal 21 to a voltage of approximately 24 volts having the polarity indicated in the drawing.

Thereafter, when the grounded input terminal 21 becomes positive with respect to the ungrounded terminal 22 during the next half cycle of the energizing voltage, a charging path is established for the second capacitor C2 of the voltage doubler circuit 24 from terminal 21 through capacitor C1, capacitor C2, diode D3 and resistor R3 to terminal 22 providing a 24 volt potential across capacitor C2 which, in addition to the 24 volt potential provided by capacitor C1, permits capacitor C2 to charge to approximately 48 volts or twice the energizing voltage applied to inputs 21 and 22. Capacitor C2, when charged provides a potential of approximately 48 volts at the cathode of the silicon controlled rectifier .32.

Thereafter, when the ungrounded input terminal 22 again becomes positive relative to the grounded terminal 21 during the next half cycle of the AC energizing voltage, a positive potential is extended from terminal 22 over resistor R3, diode D2, and resistor R5 to the gate of the silicon controlled rectifier 32. When the potential at the gate of the silicon controlled rectifier 32 becomes positive relative to the cathode of the silicon controlled rectifier 32, the silicon controlled rectifier is rendered conductive, permitting capacitor C2 to discharge through the primary winding 26 of the ignition transformer T1.

The discharge of capacitor C2 through the primary winding 26 of the ignition transformer T1 provides a current pulse in the primary winding 26 which in turn produces a voltage pulse in a secondary winding 27 of the ignition transformer T1. The voltage pulse thus produced is applied to the ignition electrodes 13 and 14, generating an ignition spark between the electrodes 13 and 14. The spark gap provided by the electrodes is placed in the region adjacent the outlet of the conduit 16. Accordingly, when the gas flow control valve 18 is open, and gas is emanating from the conduit 15, the ignition sparks thus generated will ignite the gas.

The ignition sequence with capacitor C2 of the voltage doubler network 24 being alternately charged to twice the applied energizing voltage and discharged through the primary winding 26 of the ignition transformer T1 during successive half cycles of the energizing voltage will continue, providing ignition sparks in the region of the outlet 15 of the gas burner apparatus 17 until the gas is ignited.

When a flame is present between the electrodes 13 and 14, the resistance between the electrodes decreases. Accordingly, when the gas becomes ignited, and as the ungrounded terminal 22 becomes positive with respect to the grounded terminal 21, current will flow over a unidirectional path from the ungrounded terminal 22 over resistors R3, R6 and R7, diode D5, the secondary winding 27 of the ignition transformer T1, the ignition electrodes 13 and 14 to ground. Capacitor C4 provides a high AC. voltage return path for the voltage produced in secondary winding 27, while blocking unidirectional current flow to ground. This current produces a voltage drop across resistors R6 and R7, causing the gate of the thyristor 37 to become negative with respect to the gate of the thyristor, rendering the thyristor 37 conductive, short circuiting diode D3 and providing an effective short between the gate and the cathode of the silicon controlled rectifier 32, preventing the capacitor C2 from discharging.

Consequently, spark generation will be terminated as long as the flame remains lit. Capacitor C3, which is connected in parallel with resistors R6 and R7 is charged during positive half cycles of the energizing voltage by current flow through the flame, when terminal 22 is positive relative to terminal 21, to provide a potential at the junction of resistors R6 and R7 sufficient to maintain the thyristor 37 conductive during negative half cycles of the energizing signal.

If the pilot flame suddenly becomes extinguished, such as by flame-out or sudden interruption in the gas supply, the resistance between the ignition electrodes 13 and 14 will increase, precluding current flow 3 6 through resistors R6 and R7 such that thyristor 37 will be cut off,

Consequently, the silicon controlled rectifier 32 and the voltage doubler network 24 will again be controlled by the energizing voltage supplied to the fuel ignition circuit 10 over input terminals 21 and 22 to effect the generation of ignition sparks between electrodes 13 and 14 to re-establish the flame.

In one exemplary embodiment, the resistive and capacitive circuit components for the fuel ignition circuit 10 may have the following values:

Resistor R1 450 ohms Resistor R2=R4 1000 ohms Resistor R3 100 ohms Resistor R5 470 ohms Resistor R6 5.7 Megohms Resistor R7 320K ohms Capacitor Cl 22 microfarads Capacitor C2 3.3 microfarads Capacitor C3=C4 0.0l microfarads The foregoing values are provided to illustrate one embodiment for the fuel ignition circuit provided by the present invention and are not intended as a limitation of true scope of the invention.

We claim:

1. In an electronic fuel ignition circuit for providing electronic ignition of a gaseous fuel emanating from gas burner apparatus, an ignition transformer having a primary winding and a secondary winding, voltage doubler means connected to said primary winding and including first and second capacitor means, said voltage doubler means being connectable to a voltage source which provides a cyclical AC voltage of a predetermined amplitude for, charging said first capacitor means to a voltage of said predetermined amplitude during a first half cycle of the AC voltage and charging said second capacitor means to a voltage that is twice said predetermined amplitude during a second half cycle of the AC voltage, switching means connected to said voltage doubler means and operable to provide a discharge path for said second capacitor means through said primary winding whenever said second capacitor means becomes charged to twice said predetermined amplitude, permitting capacitor discharge current to flow through said primary winding to thereby induce a voltage pulse in said secondary winding, and ignition electrode means including a pair of ignition electrodes spaced apart to provide a gap therebetween, connected in an igniter circuit with said secondary winding whereby an ignition spark is generated between said ignition electrodes whenever a voltage pulse is induced in said secondary winding, said ignition electrodes being positioned relative to said gas burner apparatus such that ignition sparks thus generated will ignite the gas emanating from said gas burner apparatus to provide a flame which bridges the gap between the ignition electrodes.

2. An electronic fuel ignition circuit as set forth in claim 1 wherein said switching means comprises a controlled switching device connected in series with said primary winding across said second capacitor means, said controlled switching device being normally nonconductive and being rendered conductive by said voltage doubler means during a first half cycle of the AC voltage after said second capacitor means becomes charged to twice said predetermined voltage.

3. An electronic fuel ignition circuit as set forth in claim 2 wherein said controlled switching device comprises a silicon controlled rectifier.

4. An electronic fuel ignition circuit as set forth in claim 1 wherein said voltage doubler means comprises a first unidirectional circuit, including said first capacitor means, connected between a first and a second input terminal of said fuel ignition circuit, and a second unidirectional circuit, including said second capacitor means, connected between said second and said first input terminals, said AC voltage being supplied to said fuel ignition circuit over said first and second input terminals to permit said first and second capacitors to be charged over said first and second unidirectional circuits, respectively, during alternate half cycles of said AC voltage.

5. An electronic fuel ignition circuit as set forth in claim 1 which includes flame detector means connected in said igniter circuit, including further switching means enabled whenever a flame bridges the gap between said ignition electrodes to inhibit operation of said switching means, thereby preventing the discharge of said second capacitor means and the generation of further ignition sparks.

6. In a spark igniter for use in an automatic fuel ignition circuit for providing electronic ignition of a gaseous fuel emanating from an outlet ofa gas burner apparatus, a pair of ignition electrodes spaced apart to provide a gap therebetween, an ignition transformer having a primary winding and a secondary winding connected in series with said ignition electrodes, blocking capacitor means connected in parallel with said secondary winding and said ignition electrodes forming an AC circuit path, said primary winding being connectable to a source of current pulses for supplying current pulses to said primary winding to thereby induce voltage pulses in said secondary winding for application to said ignition electrodes to produce ignition sparks therebetween, said ignition electrodes being positioned relative to said outlet of said gas burner apparatus for enabling ignition sparks produced between said ignition electrodes to ignite gas emanating from said outlet to provide a flame which bridges the gap between said ignition electrodes, and circuit means including resistance means and diode means connecting said secondary winding and said ignition'electrodes in a DC circuit path across a source of potential to permit unidirectional current flow over said DC circuit path whenever a flame bridges the gap between said ignition electrodes to thereby provide a DC potential over said resistance means indicative of the presence of a flame in said gap.

7. A spark igniter as set forth in claim 6 wherein said resistance means includes first and second resistors serially connected in said DC circuit path, and flame sensing capacitor means connected in parallel with said first and second resistors, said source of potential supplying a cyclical AC voltage across said DC circuit path, permitting current flow over said DC circuit path whenever a flame bridges the gap between said ignition electrodes to thereby establish a DC potential at the junction of said first and second resistors during each first half cycle of said AC voltage indicating that the gas is ignited, and to charge said flame sensing capacitor means to a voltage sufficient to maintain the DC potential at the junction of said first and second resistors 

1. In an electronic fuel ignition circuit for providing electronic ignition of a gaseous fuel emanating from gas burner apparatus, an ignition transformer having a primary winding and a secondary winding, voltage doubler means connected to said primary winding and including first and second capacitor means, said voltage doubler means being connectable to a voltage source which provides a cyclical AC voltage of a predetermined amplitude for charging said first capacitor means To a voltage of said predetermined amplitude during a first half cycle of the AC voltage and charging said second capacitor means to a voltage that is twice said predetermined amplitude during a second half cycle of the AC voltage, switching means connected to said voltage doubler means and operable to provide a discharge path for said second capacitor means through said primary winding whenever said second capacitor means becomes charged to twice said predetermined amplitude, permitting capacitor discharge current to flow through said primary winding to thereby induce a voltage pulse in said secondary winding, and ignition electrode means including a pair of ignition electrodes spaced apart to provide a gap therebetween, connected in an igniter circuit with said secondary winding whereby an ignition spark is generated between said ignition electrodes whenever a voltage pulse is induced in said secondary winding, said ignition electrodes being positioned relative to said gas burner apparatus such that ignition sparks thus generated will ignite the gas emanating from said gas burner apparatus to provide a flame which bridges the gap between the ignition electrodes.
 2. An electronic fuel ignition circuit as set forth in claim 1 wherein said switching means comprises a controlled switching device connected in series with said primary winding across said second capacitor means, said controlled switching device being normally non-conductive and being rendered conductive by said voltage doubler means during a first half cycle of the AC voltage after said second capacitor means becomes charged to twice said predetermined voltage.
 3. An electronic fuel ignition circuit as set forth in claim 2 wherein said controlled switching device comprises a silicon controlled rectifier.
 4. An electronic fuel ignition circuit as set forth in claim 1 wherein said voltage doubler means comprises a first unidirectional circuit, including said first capacitor means, connected between a first and a second input terminal of said fuel ignition circuit, and a second unidirectional circuit, including said second capacitor means, connected between said second and said first input terminals, said AC voltage being supplied to said fuel ignition circuit over said first and second input terminals to permit said first and second capacitors to be charged over said first and second unidirectional circuits, respectively, during alternate half cycles of said AC voltage.
 5. An electronic fuel ignition circuit as set forth in claim 1 which includes flame detector means connected in said igniter circuit, including further switching means enabled whenever a flame bridges the gap between said ignition electrodes to inhibit operation of said switching means, thereby preventing the discharge of said second capacitor means and the generation of further ignition sparks.
 6. In a spark igniter for use in an automatic fuel ignition circuit for providing electronic ignition of a gaseous fuel emanating from an outlet of a gas burner apparatus, a pair of ignition electrodes spaced apart to provide a gap therebetween, an ignition transformer having a primary winding and a secondary winding connected in series with said ignition electrodes, blocking capacitor means connected in parallel with said secondary winding and said ignition electrodes forming an AC circuit path, said primary winding being connectable to a source of current pulses for supplying current pulses to said primary winding to thereby induce voltage pulses in said secondary winding for application to said ignition electrodes to produce ignition sparks therebetween, said ignition electrodes being positioned relative to said outlet of said gas burner apparatus for enabling ignition sparks produced between said ignition electrodes to ignite gas emanating from said outlet to provide a flame which bridges the gap between said ignition electrodes, and circuit means including resistance means and diode means connecting said secondary windiNg and said ignition electrodes in a DC circuit path across a source of potential to permit unidirectional current flow over said DC circuit path whenever a flame bridges the gap between said ignition electrodes to thereby provide a DC potential over said resistance means indicative of the presence of a flame in said gap.
 7. A spark igniter as set forth in claim 6 wherein said resistance means includes first and second resistors serially connected in said DC circuit path, and flame sensing capacitor means connected in parallel with said first and second resistors, said source of potential supplying a cyclical AC voltage across said DC circuit path, permitting current flow over said DC circuit path whenever a flame bridges the gap between said ignition electrodes to thereby establish a DC potential at the junction of said first and second resistors during each first half cycle of said AC voltage indicating that the gas is ignited, and to charge said flame sensing capacitor means to a voltage sufficient to maintain the DC potential at the junction of said first and second resistors during each second half cycle of said AC voltage. 